Simplify cancel_shielded_checkpoint ?

[touilleMan]

I'm not sure what is the need for a mix of yield and asyncfunction decorator here:

trio/trio/_core/_traps.py

Lines 13 to 45 in a351a2d

	def asyncfunction(fn):
	# Set the coroutine flag
	fn = types.coroutine(fn)
	# Then wrap it in an 'async def', to enable the "coroutine was not
	# awaited" warning
	@wraps(fn)
	async def wrapper(*args, **kwargs):
	return await fn(*args, **kwargs)
	return wrapper
	# This class object is used as a singleton.
	# Not exported in the trio._core namespace, but imported directly by _run.
	class CancelShieldedCheckpoint:
	pass
	@asyncfunction
	def cancel_shielded_checkpoint():
	"""Introduce a schedule point, but not a cancel point.
	This is *not* a :ref:`checkpoint <checkpoints>`, but it is half of a
	checkpoint, and when combined with :func:`checkpoint_if_cancelled` it can
	make a full checkpoint.
	Equivalent to (but potentially more efficient than)::
	with trio.open_cancel_scope(shield=True):
	await trio.hazmat.checkpoint()
	"""
	return (yield CancelShieldedCheckpoint).unwrap()

From what I understand, it is because an awaited object must be of a type with an __await__ method.
But in such a case there could be a much simpler wait to do this:

class CancelShieldedCheckpointType:
    def __await__(self):
        yield self
CancelShieldedCheckpoint = CancelShieldedCheckpoint()
del CancelShieldedCheckpointType

async def cancel_shielded_checkpoint():
    return await CancelShieldedCheckpoint

This way CancelShieldedCheckpointType is just a helper class (hence destroyed as soon at it is no longer needed) and CancelShieldedCheckpoint is still a singleton, this time compatible with await syntax.

[njsmith]

Hmm, you're right, that code does look kinda overcomplicated. I'd rather not create __await__ methods just for this, but what do you think of #397?

[touilleMan]

Looks good to me 👍

[njsmith]

Want to click merge, then? :-)

[touilleMan]

That would be an honor !

[njsmith]

Thanks! BTW, sorry I haven't followed up with pytest-trio stuff recently -- I'm hoping to get back to that soon, maybe this weekend.